class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root==null) return ret;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        int count=1;
        while(!q.isEmpty()){
            int sz =q.size();
            List<Integer> tmp = new ArrayList<>(); 
            for(int i=0;i<sz;i++){
                TreeNode t = q.poll();
                tmp.add(t.val);
                if(t.left!=null) q.add(t.left);
                if(t.right!=null) q.add(t.right);
            }
            if(count%2==0) Collections.reverse(tmp);
            ret.add(tmp);
            count++;
        }
        return ret;
    }
}